1. Field of the Invention
The present invention relates to an optical pickup apparatus performing a reading operation of a signal recorded in an optical disc or a recording operation of a signal into the optical disc.
2. Description of Related Art
There are widely used optical disc devices capable of a signal reading operation and signal recording operation by applying laser light emitted from an optical pickup apparatus to a signal recording layer of an optical disc.
While there are widely available in general the optical disc devices using optical discs called CD and DVD, there have recently been developed those using optical discs improved in recording density, such as optical discs of a Blu-ray standard and an HD DVD (High Density Digital Versatile Disc) standard.
As laser light for performing the reading operation of a signal recorded in a CD standard optical disc, infrared light with a wavelength of 780 nm is used, and as laser light for performing the reading operation of a signal recorded in a DVD standard optical disc, red light with a wavelength of 650 nm is used.
Thickness of a protective layer provided on a top face of a signal recording layer of the CD standard optical disc is specified at 1.2 mm, and a numerical aperture of an objective lens used for the reading operation of a signal from this signal recording layer is specified at 0.45. Thickness of a protective layer provided on a top face of a signal recording layer of the DVD standard optical disc is specified at 0.6 mm, and a numerical aperture of an objective lens used for the reading operation of a signal from this signal recording layer is specified at 0.6.
Laser light with a short wavelength, such as blue violet light with a wavelength of 405 nm is used as the laser light for performing the reading operation of a signal recorded in an optical disc of Blu-ray standard and the HD DVD standard, as compared with the laser light for the reading operation in the case with such CD standard and DVD standard optical discs.
Thickness of a protective layer provided on a top face of a signal recording layer of the HD DVD standard optical disc is specified at 0.6 mm, and a numerical aperture of an objective lens used for the reading operation of a signal from this signal recording layer is specified at 0.65.
As described above, for the optical discs of different standards, since the laser lights for performing the reading operations of signals thereof are different in wavelength, in order to perform the reading operation of signals from all the optical discs, a configuration is made such that a laser diode emitting laser lights having different wavelengths is used (see Japanese Laid-Open Patent Publication No. 2004-295983).
To perform the reading operation of signals recorded in all the optical discs, since locations of the signal recording layers are different, it is required to change a numerical aperture corresponding to each of the optical discs. Therefore, there is developed an optical pickup apparatus capable of performing such an operation (see Japanese Laid-Open Patent Publication No. 2006-172605).
When performing the reading operation of signals recorded in optical discs by using the same objective lens and laser lights having different wavelengths, the respective focal points of the laser lights are different, and for this reason, there is developed an art using an objective lens having an annular diffraction grating formed on its incident face to focus laser lights having different wavelengths to the optical discs of different standards (see Japanese Laid-Open Patent Publication No. 2000-81566).
When performing the reading operation of signals recorded in optical discs of different standards, not only are there required a laser diode emitting the laser light having the wavelength corresponding to each of the optical discs and means for setting the numerical aperture of the objective lens corresponding to each of the optical discs, but also it is required to provide an optical component for each of the optical discs. Therefore, there are problems that a configuration becomes complicated, and moreover, production costs become high.